This is a Reschke
wind-driver generator propeller or Hilfspropeller.
It was typically mounted onto a generator which was attached to
an undercarriage strut or leg. The generator and propeller
were positioned such that the wind stream generated by the
aircraft's main propeller and the forward momentum of the
aircraft in flight would drive the little Hilfspropeller
to turn and by so turning the propeller would force the
generator to create an electrical current which in turn would
power equipment on the aeroplane, such as heated suits for
flightcrew.

Below you can see
the metal hub removed.

Here is a close-up of the hub on the left
and one of the Reschke company transfers or decals on the right.

On the left are two
illustrations from Flugsport showing the propeller mounted on a
wind-driven generator and on the right is a copy of a Reschke
company ad from Flugsport..